The present invention relates generally to the control of corrosion in steam generating equipment, and more particularly, to an improved boiler feedwater conditioning method for removing dissolved oxygen and passivating metal surfaces.
The treatment of water for use in steam generating equipment is a very critical and complex art due to the numerous sources of scaling, corrosion and other water related problems typically encountered in operating such equipment. This invention is concerned with a method of conditioning feedwater to protect preboiler, boiler and condensate systems of steam generating equipment against corrosion during operation and lay-up.
The most common source of corrosion in such systems is oxygen attack of steel components. Unfortunately, oxygen attack of steel is accelerated by the unavoidably high temperatures found in boiler equipment. Also, if boiler water pH is permitted to become acidic (which helps control scale formation), oxygen attack is yet further accelerated.
In most modern steam generating systems, dissolved oxygen levels are controlled by first mechanically removing the bulk of the dissolved oxygen and then chemically scavenging the remainder. Mechanical degasification is typically carried out with vacuum degasifiers which reduce oxygen levels to less than 0.5-1.0 mg/l or deaerating heaters, which reduce oxygen concentrations to 0.005-0.01 mg/l.
Traditionally, sodium sulfite and hydrazine have been used to chemically scavenge the oxygen remaining in steam generating systems after the initial mechanical removal of the bulk of the dissolved oxygen. Each of these traditional treatments has significant shortcomings.
Sodium sulfite, for example, is not recommended for use in systems operating above 1500 psi because corrosive hydrogen sulfide and sulfur dioxide can be formed at pressures above this point. Also, sodium sulfite can contribute to increased dissolved solids in the feedwater, requiring higher boiler blowdown rates and, therefore, higher water, fuel and chemical costs.
Hydrazine is less effective than sulfite in removing oxygen. However, since hydrazine also acts as a corrosion inhibitor by maintaining a passive, protective film on system components, it is an effective alternative to sulfite. Unfortunately, though, hydrazine is a toxic substance which must be handled with extreme care in all applications. Indeed, Food and Drug Administration rules prohibit the presence of measurable quantities of hydrazine in any applications in which it might come in contact with food.
It is, therefore, an object of the present invention to provide an improved method for scavenging oxygen in steam generating systems which relies on neither sulfite nor hydrazine.
It is a further object of the present invention to provide a feedwater conditioning method for passivating metal surfaces in steam generating equipment without relying on hydrazine.
Other objects and advantages of the present invention will become apparent from the discussion below.